#30 namespace lookup sometimes fails

[Donal K. Fellows]

With some DOM implementations,
SOAP::Utils::file_namespaceURI fails. This is because
instead of [dom::node parent $documentNode] returning
the empty string, it returns something else (can't
remember if it is an error or an invalid string). This
can be fixed by not recursing if you're at the document
root (which can be done early anyway; document roots
don't have attributes).

[Pat Thoyts]

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[11:27]<dkf> I was just trying to run the code to tool up
the Google API from WSDL
[11:27]<dkf> and it was giving a very odd error message
[11:28]<dkf> Error message was: token "malformed node token
"doc0"" is not a DOM Document
[11:29]<dkf> looks like an error message didn't get marked
as an error... :/
[11:29]<dkf> but since the workaround was so easy anyway, it
seemed easier to report as a fault